1. Field of the Invention
The present invention relates to a liquid crystal display with the alignment of the ferroelectric liquid crystal controlled and a manufacturing method of the same.
2. Description of the Related Art
Since liquid crystal displays have features that it is thin and is low in power consumption and other features, the use thereof has been expanding in various articles from large-sized displays to portable information terminals and the development thereof has been actively made. Conventionally, for liquid crystal displays, a TN system, an STN multiplex driving system, an active matrix driving system in which thin film transistors (TFTs) are used in TN, and others have been developed and made practicable. However, since nematic liquid crystal is used therein, the response speed of the liquid crystal material is as small as several milliseconds to several tens of milliseconds and it cannot be said that these sufficiently cope with display of moving images.
Ferroelectric liquid crystal (FLC) exhibits a very fast response in order of microseconds, and thus FLC is a liquid crystal suitable for high-speed devices. About ferroelectric liquid crystal, there is well known a ferroelectric liquid crystal, there is well known a bistable liquid crystal which has two stable states when no voltage is applied thereto and is suggested by Clark and Lagerwall (FIG. 7). However, the liquid crystal has a problem that the liquid crystal has memory property but gray scale display cannot be attained since the switching thereof is limited to two states, namely, bright and dark states.
In recent years, attention has been paid to ferroelectric liquid crystal in which the liquid crystal layer thereof is stable in a single state (hereinafter referred to as “monostable”) when no voltage is applied thereto as a liquid crystal making it possible to attain gray scale display by the matter that the director (the inclination of the molecule axis) of the liquid crystal is continuously changed by a change in applied voltage so as to analogue-modulate the light transmission thereof (NONAKA, T., LI, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599, FIG. 7). As the ferroelectric liquid crystal showing the mono-stability, there are a material having the phase change of cholesteric phase (Ch)-chiral smectic C phase (SmC*) without the transition to the smectic A (SmA) phase in the temperature lowering process and a material having the phase change of Ch-SmA-SmC* so as to show the SmC* phase via the SmA phase in the temperature lowering process (FIG. 8).
Ferroelectric liquid crystal has a higher order of molecules therein than nematic liquid crystal; therefore, the former liquid crystal is not easily aligned so that defects called zigzag defects or hairpin defects are easily generated. Such defects cause a fall in contrast based on light leakage.
Moreover, the ferroelectric liquid crystal undergoing phase transition via no SmA phase in the phase sequence generates two domains different in the layer normal-line direction thereof (hereinafter referred to as “double domains”) (FIG. 8). The double domains give such display that black and white are reversed when driven so as to cause a serious problem (FIG. 9). As the method for removing the double domains, known is an electric field induced technique (, which uses DC voltage during cooling process) of heating a liquid crystal cell to a temperature not lower than the Ch phase thereof, and then cooling the liquid crystal cell slowly while applying a DC voltage thereto (PATEL, J., and GOODBY, J. W., 1986, J. Appl. Phys., 59, 2355). This method has problems that the alignment of the liquid crystal is disturbed when the temperature thereof is again raised to a temperature not lower than the phase transition temperature thereof and the alignment is disturbed in regions where no electric field acts between pixel electrodes, and other problems.
On the other hand, since the ferroelectric liquid crystal having the SmA phase in the phase sequence in general has two stable states for one layer normal so as to show the bi-stability, it is difficult to obtain a mono-stability state.
In general, as the technique for subjecting liquid crystal to aligning treatment, there is known a method of using an alignment layer. The method is classified into the rubbing method and the photo aligning method. The rubbing method is a method of subjecting a substrate coated with a polyimide film to rubbing treatment to align chains of the polyimide polymer in the direction of the rubbing, thereby aligning liquid crystal molecules on the layer. The rubbing method is excellent in controllability of the alignment of nematic liquid crystal, and is generally an industrially applicable technique. On the other hand, the photo aligning method is a method of radiating light the polarization of which is controlled onto a polymer or a monomolecular to generate photo-excited reaction (decomposition, isomerization or dimerization) so as to give anisotropy to the polymer film, thereby aligning the liquid crystal molecules on the film. However, it is difficult to obtain the mono-stable ferroelectric liquid crystal alignment with either method.
Moreover, although it does not provide the mono-stability, as a method for reducing the alignment defect of the ferroelectric liquid crystal, the official gazette of the Japanese Patent Application National Publication (Laid-Open) No. 2002-532755 discloses a method for aligning the ferroelectric liquid crystal by forming a nematic liquid crystal layer by coating, aligning and fixing a nematic liquid crystal on each alignment layer after applying the photo alignment treatment to upper and lower alignment layers, and functioning the nematic liquid crystal layers as an alignment layer. However, the method is not for restraining generation of the alignment defect of the ferroelectric liquid crystal having the mono-stability.
On the other hand, in recent years, color liquid crystal displays have been actively developed. The method for realizing color display is generally classified into a color filter system and a field sequential color system. The color filter system is a system of using a white light source as a back light and attaching a micro color filter in R, G or B color to each pixel, thereby realizing color display. On the other hand, the field sequential color system is a system of switching a back light into R, G, B, R, G, B . . . with time, and opening and shutting a black and white shutter of a ferroelectric liquid crystal in synchronization therewith to mix the colors with time by afterimage effect on the retina, thereby realizing color display. This field sequential color system makes it possible to attain color display in each pixel, and does not require any color filter low in transmission. As a result, this system is useful since the system is capable of attaining bright and highly precise color display and realizing low power consumption and low costs. However, the field sequential color system is a system in which each pixel is subjected to time sharing; it is therefore necessary for the liquid crystal as the black and white shutter to have high speed response property in order to give a good moving image display property. If ferroelectric liquid crystal is used, this problem can be solved. Nonetheless, the ferroelectric liquid crystal has a problem that alignment defects are easily generated, as described above, and thus the color system using this liquid crystal has not been made practicable.